The present invention relates to apparatus for and method of diagnosing a combustion state of multi-cylinder engine with a change of revolution speed among the cylinders.
Abnormality of combustion in the engine such as occurrence of misfire results in atmosphere contamination because of exhausting unburnt gases. Further, in catalyzer, etc. provided to purify the exhaust gas, when the unburnt gas burns, an exhaust purifying apparatus section thereof is heated to an abnormally high temperature, and its performance is degraded. For preventing this, it was necessary to detect misfire occurrence to send an driver an alarm, and to stop fuel supply to a cylinder in which misfire was occurring.
There are many prior arts concerning diagnosis apparatus of a combustion state such as misfire. Some examples of them are a method of detecting it with change in revolution speed of the engine, a method of detecting it using combustion pressure, temperature, etc. in a combustion chamber, a method of detecting a combustion state using a change in revolution speed among the cylinders, etc. Of the various methods, the method of detecting abnormality of combustion state, using a change in revolution speed has a characteristic that the combustion state can be detected with a relatively small cost increase and irrespective of their causes (even if deterioration in combustion state is caused by abnormality in any of fuel line, ignition line and air line). This method, as is disclosed in JP A 2-112646 and JP A 1-30098, detects revolution speed at a particular crank angle for each cylinder since the rotational speed differs between normal combustion and abnormal combustion, and diagnoses the combustion state on the basis of a change in the revolution speed among the cylinders.
Further, in order to remove a change in revolution speed due to outer disturbance factors such as a rough road other than engine misfire, a method of discriminating a specific operation state in which it is predictable that many such outer disturbances are inputted, on the basis of information of a remaining fuel amount, a brake switch, automobile speed, etc., and not executing misfire detection processing during such a specific operation period is disclosed in JP A 4-203252. Further, a method of using information of each wheel speed, detecting whether or not there is outer disturbance input on the basis of signals from an acceleration meter mounted on a car body, and stopping misfire judgment is disclosed in JP A 4-209950, JP A 4-295161, etc.
However, stopping all the misfire judgment at the above-mentioned specific operation detracts from preciseness in the misfire judgment. For example, even at a time of high speed rotation, at an operation of the brake switch, it is necessary that normal combustion and misfire can be judged correctly. Further, the above-mentioned method using wheel speed sensor information is necessary to mount revolution speed detection sensor on each wheel, and it was difficult to make a correlation between wheel speed change level and engine revolution change occurrence. On the other hand, in a method of stopping misfire judgment by detecting vibrations caused on the car body on a rough road by an acceleration sensor, such an acceleration sensor must be provided, it is difficult to make a correlation between engine revolution change starting and acceleration, and it takes much time to set the control data.